arm7tdmi: clean up architecture definition

src/runtime/arch_arm.go

@@ -1,7 +1,9 @@
-// +build arm,!baremetal
+// +build arm,!baremetal arm,arm7tdmi
 
 package runtime
 
+import "device/arm"
+
 const GOARCH = "arm"
 
 // The bitness of the CPU (e.g. 8, 32, 64).
@@ -11,3 +13,7 @@ const TargetBits = 32
 func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3
 }
+
+func getCurrentStackPointer() uintptr {
+	return arm.ReadRegister("sp")
+}